ABSTRACT
Diarthrosis, a synovial joint, which provides a flexible unification of the skeleton bone elements, comprise four structural components: 1) articular cartilages; 2) a joint capsule and intra-articular ligaments; 3) a synovial membrane; and 4) synovia (synovial fluid). The biochemical and molecular biological characteristics of articular cartilages, which are typically hyaline cartilaginous tissue, were already described in Chapter 7. The peculiarities of the special subtypes of intra-articular ligaments in some joints, meniscs and disks, composed of fibrous cartilaginous tissue, have also been discussed therein. The joint capsule and intra-articular (and periarticular) ligaments uniting bones to other bones are made of dense formatted fibrous connective tissue, the biochemical and molecular biological features of which are outlined in section 3.5.3. The synovial membrane (synovial coat) – the fine inner layer of the joint capsule composed of loose non-shaped fibrous connective tissue – borders the joint cavity on one side, and, on the other, turns into dense fibrous connective tissue of the outer, thicker layer of the capsule. There is no synovial membrane on the uncovered intraarticular surfaces of the inner articular cartilages. It is of significance that, from the side of the joint cavity, neither the synovial membrane nor the articular cartilages have any special border-line coating. The synovial membrane is one of varieties of connective tissue where the cell elements quantitatively predominate the extracellular matrix, particularly due to the surface layer. The specialized (covering) cells of the membrane are referred to as synoviocytes, with two known types: type A synoviocytes and type B synoviocytes. These cells do not constitute a continuous layer and there are areas of uncovered extracellular matrix between them. Besides synoviocytes, the membrane contains an insignificant number of histiocytes, plasma cells, mast cells (heparinocytes), lymphocytes and blood macrophages. Type A synoviocytes are mobile and are capable of actively phagocytizing cell wastes from the synovia. They express antibodies and some cytokines (interleukins). The type A cells originate from blood mononuclear cells and are considered as tissue (resident) macrophages, unlike blood macrophages. They are also referred to as macrophage-like synoviocytes. The type B synoviocytes are characterized by an intensive development of the granular endoplasmic reticulum and the presence of branching processes that form a network on an inner (inward to the joint cavity) membrane surface. Having significant biosynthetic activity, these cells produce the main macromolecular components of the synovia (in particular, hyaluronan) and the synovial membrane extracellular matrix. Type B synoviocytes are a specialized kind of fibroblast in terms of
their morphological and functional characteristics (Iwanaga et al., 2000). Sometimes they are called synovial fibroblasts (SF). The synovial membrane has no clear-cut outer boundary and its fineness can account for the practical inability to its precise preparative separation. This also makes it difficult to carry out a biochemical study of the membrane. Therefore, a significant part of the findings on the biochemical characteristics of the membrane has been obtained by means of various histochemical and cytochemical methods, and this data are not quantitative. The advantage of this data is that it makes it possible to evaluate the distribution of the components revealed according to the membrane layers – an inner layer, directly adjoining the synovia (intimal), and a deeper (subintimal, collagen-elastic) layer. At the same time, this data is also significantly contradictory; this divergence might be due to the differences between membrane layers, which were not always taken into account when studying the entire membrane, as well as interspecific differences inherent to the synovial membrane (Revell et al., 1995). The collagen-elastic layer is comprised of a network of fine collagen fibers, characterized by argyrophily (impregnated with silver), which indicates a deposition of glycoproteins on the surface of these fibers, predominately consisting of type III collagen. These fibers are defined as reticular (Ushiki, 2002). Data on collagen components of the synovial membrane are rather inconsistent. On the one hand, there are reports that the synovial membrane of rabbits contains type III, V and VI collagen, with the quantitative prevalence of type III collagen forming the above-mentioned network of reticulin fibers. According to these data, type III collagen makes up more than 50% of the total collagen mass, which significantly exceeds this collagen content in other connective tissue varieties in adults. The formation of a network of microfibrils (with a diameter of about 9 nm) have also been described (in rabbits as well). This network is located within the intimal layer at a depth of 2-3 µm from the surface i.e. it can be referred to a surface collagen-elastic complex (Levick & McDonald, 1990). The ultrastructural characteristics of these microfibrils correspond to the properties of type VI collagen; they are not like fibrillin microfibrils. The expression of type VI collagen by cultured synovial membrane cells has also been detected (Wolf & Carsons, 1991). Both in vivo and in an extracellular matrix produced by the culture, type VI collagen is found in the surface (intimal) layer, together with fibronectin, which generally indicates its expression by synoviocytes located nearer to the surface and a certain functional relationship between these macromolecular components. Type VI collagen is also found in the walls of synovial membrane blood vessels. Other immunohistochemical investigations of the human synovial membrane intima (Revell et al., 1995) have confirmed the presence of only type V and IV collagens in this layer (the latter shall be discussed separately). It
may be supposed that type III collagen is concentrated in a deep collagen-elastic layer, and whether it is present in the intima of the human synovial membrane remains unclear. One of the specific peculiarities of synovial membrane collagens is the presence of a glucosylgalactosylpyridinoline non-reducib-le cross-link (Glc-Gal-PYD) (Garnero et al., 2001). Bone and cartilage collagens have no such cross-link; the appearance of peptides containing this link in urine in joint disorders might be considered as an indicator of the destruction of synovial membrane collagens and of its involvement in a pathological process. Together with collagen fibers, the collagen-elastic layer contains the fibers stained with orcein – a staining substance specific for elastic fibers. There is a lack of any information about the biochemical properties of the synovial membrane elastin. The presence of hyaluronan in significant amounts within the surface layer is the most essential peculiarity of the in-terfibrillar matrix of the synovial membrane (Worrall et al., 1991), which is absolutely natural in terms of the immediate contiguity of the membrane and the synovial f luid. It is the synovia where the hyaluro-nan concentration is especially high. In a healthy human, synovial membrane hyaluronan is mainly found around synoviocytes in the surface layer, with lower concentration in deeper layers. Proteoglycans, containing chondroitin 4-sulfate, chondroitin 6-sulfate, keratan sulfate and heparin sulfate as glycosaminoglycan components, have been shown to be present in the synovial membrane. According to research findings in rabbits whose synovial membranes contained the corresponding core proteins, chondroitin sulfate was found to be present in biglycan and decorin macromolecules, while keratan sulfate was found in fibromodulin (Coleman et al., 1998). According to immunohistochemical findings, there is a very thin layer of fibronectin macromolecules in the most superficial layer of a mouse’s synovial membrane (Linck et al., 1983). This suggests that fibronectin plays an important functional role in the vital activities of the synovial mem-brane. This suggestion is confirmed by a biosynthesis of fibronectin by the cultured sections of a normal human membrane (Lavietes et al., 1985). Upon being synthesized in such a culture, fibronectin was not only secreted into the cultural medium but also embedded into a resulting matrix. It is probable that both type A and type B synoviocytes take part in fibronectin production. It may be assumed that interacting with a network of collagen fibers and proteoglycan components of the extracellular matrix fibronectin forms a special layer on the border between the synovial membrane and synovial f luid. This layer controls the exchange of substances between these two media. One of tenascins, tenascin-X, appears to be an obligatory component of the normal synovial membrane (Li et al.,2000). It is also found in a synovial pseudomembrane, developed around loosening joint prostheses, and it is of probable impor-
tance for stabilizing the supramolecular structural organization of the membrane extracellular matrix. Macromolecules known as basement membrane components (the above-mentioned type IV collagen and laminins) have been detected in synovial membrane tissue, particularly around blood vessels, using immunohistochemical methods. This is of particular interest in connection with the lack of morphologically structured basement membranes within the synovial membrane. There are basement membranes neither inside nor outside the membrane, neither between the membrane layers, nor around blood vessels (Revell et al., 1995). Type IV collagen is represented by a limited set of isoforms, mainly, α1(IV) and α2(IV) polypeptide chains, together with a few α5(IV) and α6(IV) chains within the synovial membrane. This is the difference between the synovial membrane and the tissues, with formed basement membranes containing all of the six known isoforms. The mRNA of type IV collagen isoforms of the membrane has been found in synovial fibroblasts (type B synoviocytes). It is assumed that components of the basement membrane in the synovial membrane play a certain organizing role in the extracellular matrix structure. The fact that the abrupt decrease in type IV collagen observed in rheumatoid arthritis is accompanied by the increased permeability of the synovial membrane for white blood cells favors this hypothesis (Poduval et al., 2007). As in other types of connective tissue, all the structural components of the extracellular matrix in the synovial membrane are expressed and secreted by fibroblasts and, in the given case, by specialized synovial fibroblasts (type B synoviocytes, SF). The same cells are producers of macromolecular components, enabling interaction between the synovial membrane cells. These components are the adhesive molecules of cell membranes (VCAM-1, ICAM-1) and desmosome proteins, as well as two cadherins – cadherin 11 and N-cadherin – specific for mesenchymal cells (see section 3.6.3). Each of these cadherins acts as a part of separate supramolecular complexes within the synovial membrane (Agarwal et al., 2008). Type B synoviocytes produce not only components of the synovial membrane extracellular matrix but also specific components of the synovial fluid – proteins that differ from blood plasma proteins – as well as hyaluronan. In vitro studies demonstrate that the synthesis of hyaluronan is controlled by pro-and anti-inflammatory cytokines (Hyc et al., 2009). Along with the structural components, the synovial fibroblasts express enzymes that participate in metabolic processes within the synovial membrane, synovia and articular cartilages. Metalloproteases of the ADAMTS family (see section 3.7.1) are among these enzymes. ADAMTS-enzymes, particularly ADAMTS-5, which is responsible for the catabolism of aggrecan, a large proteoglycan of cartilaginous tissue, enters articular cartilage through the synovia (Vankemmelbeke et al., 2001). Cathepsin K, expressed by synovial fibroblasts, is engaged in
the destruction of the collagens of articular cartilages during pathological processes (Hou et el., 2001). Apart from the biosythesis of the macromolecular components of its own extracellular matrix and specific components of synovia, and the opposite processes of the catabolism and resorption of the synovia, the synovial membrane serves the function of controlling substance exchange (diffusion) between blood plasma and the synovia. This exchange depends on two factors. The first factor is constituted by the physical and chemical features of the synovia (synovial fluid) itself due to the high concentration of high-polymeric glycosaminoglycan, hyaluronan, therein. This factor, conditioning the high viscosity of the synovial fluid, creates special circumstances for the bilateral diffusion of molecules through the synovial membrane. In vitro experiments demonstrate that the greater the hyaluronan’s molecular mass (and respectively the longer its macromolecules are), the more significant impact that this polymer will have on the physical and chemical conditions of molecule diffusion (Coleman et al., 2000). With a molecular mass of 530 kDa and more, hyaluronan becomes a buffer, restraining the fluid release from the synovia into the interstitial spaces of the synovial membrane. The second factor is the structure of the synovial membrane. As previously mentioned, it lacks structured basement membranes, as a result of which there are no other barriers except for the capillary endothelium, not tightly adjoining cells of the synovial membrane (synoviocytes), and the extracellular matrix that is located between the circulating blood and the synovia. This means that there is actually no barrier for low-molecular substances to penetrate the synovial fluid from the blood plasma and back. In fact, under normal conditions, the concentrations of electrolytes, glucose, uric acid, low-molecular antibiotics and other small molecules in the synovial fluid differ only a little from their concentrations in blood. However, a detailed investigation into the kinetics of this balance have shown that this process cannot be reduced to only the physical diffusion of molecules through a membrane permeable for them. In fact, the transsynovial exchange of a number of the molecules studied appeared to be different from that which was expected based on their molecular mass. For instance, the rate for the transsynovial exchange of water, studied in a normal human knee joint using tritium marking, is significantly lower in comparison with the ideal rate for diffusing molecules of this size. It appears that a molecule’s traffic through the synovial membrane is subject to more complicated regularities in relation to metabolic processes taking place in the membrane and synovia.
